The present invention relates to an inorganic/organic insulative coated semiprocessed or fully processed nonoriented electrical steel providing a high level of surface resistivity to minimize interlaminar power losses. More particularly, the insulative coating is formed from an acid aqueous suspension including a monoaluminum phosphate solution, at least one inorganic particulate silicate and an organic acrylic latex. The insulative coated nonoriented electrical steels of this invention may be used to minimize power losses in magnetic cores of motors, generators, transformers, and other electrical equipment.
Nonoriented electrical steel sheets used for medium and large size motors, generators and transformers require an insulative coating that provides a high level of surface resistivity to minimize interlaminar power losses when laminations formed from the steel are used in an as-punched condition. This high level of surface resistivity may also be required after heat treatments such as stress-relief annealing (SRA) or "burn-off" treatments used to remove winding insulation during motor repair. The insulative coating must have good adhesion to the steel sheet during manufacturing processes so that the coating does not flake from the edges of laminations punched from the steel sheet. The insulative coating also should not powder excessively, should not buildup on feed rolls and other equipment during punching, slitting or shearing, should not abrade dies used to punch or shear the laminations, should allow welding of magnetic cores formed from the laminations at reasonable speeds, and should be compatible with various chemicals and resins used during manufacture or use of the magnetic cores.
It is known to use inorganic phosphate insulative coatings for electrical steel. Inorganic phosphate coatings provide excellent surface resistivity and heat resistance but cause excessive die wear during punching of laminations and tend to have poor adhesion to the steel sheet. For example, U.S. Pat. No. 2,501,846 discloses forming an insulating film from a phosphoric acid solution containing about 7-50% free acid or a solution consisting of phosphoric acid in which magnesia has been dissolved. U.S. Pat. No. 2,743,203 discloses an inorganic phosphate insulative coating for electrical steel formed from a phosphoric acid solution in which aluminum hydrate has been dissolved. U.S. Pat. No. 3,948,786 relates to an improved inorganic phosphate insulative coating for grain oriented electrical steel formed from a solution containing 100 parts by weight of an aluminum-magnesium-phosphate solution, up to 150 parts by weight colloidal silica, and up to 25 parts by weight of chromic anhydride. These prior art inorganic phosphate insulative coatings all cause increased die wear during punching of laminations compared to punching of bare electrical steel. Also, adhesion of these coatings to the steel sheet surface is very dependent upon the nature of the oxide film formed on the surface of the sheet in the operations preceding coating.
It also is known to use inorganic insulative coatings containing an organic resin. The organic resin provides lubricity to the cured coating which reduces the rate of die wear during punching of the coated electrical steel. Inorganic phosphate coatings containing an organic resin provide excellent surface resistivity but may powder excessively during punching, slitting or shearing of the steel and cause build-up of powdered coating on the punching, slitting or shearing equipment. During periods of high humidity, inorganic/organic phosphate coatings may absorb water vapor from the atmosphere and become sticky, causing the coating to build up on feed rolls, dies, tension pads, and other equipment. The accumulated coating must be removed from the equipment by cleaning, thereby impacting productivity. U.S. Pat. No. 4,496,399 relates to an inorganic/organic phosphate insulative coating. This patent discloses an aqueous composition for coating nonoriented electrical steels with the inorganic portion of the composition including 100 parts aluminum-magnesium-phosphate and either 33-250 parts colloidal silica and 10-25 parts chromic anhydride or 30-250 parts of a particulate aluminum silicate. The organic portion of the composition contains 15-1350 parts of an aqueous suspension containing 40-60% by weight solids of an acrylic or vinyl acetate resin. A disadvantage of this coating is that the inorganic portion contains free phosphoric acid which must be reacted with the steel surface at a high temperature and also magnesium phosphate compounds which must be cured at a high temperature to prevent stickiness caused by the presence of unreacted phosphoric acid or hygroscopic phosphate compounds. At this high curing temperature, however, degradation of the resin occurs which results in a non-uniform brown appearance and poor adhesion to the steel sheet. Another disadvantage is that the coating is difficult to apply uniformly across the width of the steel sheet using grooved rubber metering rolls due to occurrence of streaks of thin coating.
U.S. Pat. No. 3,793,073 discloses an inorganic/organic insulative coating for coating electrical steel having a surface roughness of at least 20 Hr.m.s. .mu.inch. The organic portion of the coating is formed from the group of acrylic acid resins or copolymers, anhydrous maleic acid, amino acid resin, calcium lignin sulfonate, polyvinyl alcohol, phenol resin, vinyl acetate, polyvinyl acetal, alkyd resins, vinyl chloride and epoxy resins. Organic particulate materials such as bakelite, melamine resin or the like having a particle size greater than 2 .mu.m may be used to provide the surface roughness. The inorganic portion of the coating may include one or more of a phosphoric acid type material or a chromic acid type material. The phosphoric acid type material may include a phosphate of calcium, aluminum, magnesium and zinc as well as titanium oxide, colloidal silica, colloidal alumina and boric acid. The chromic acid type material may include a bichromate of calcium, magnesium, and zinc as well as titanium oxide, colloidal silica, colloidal alumina, boric acid and an organic reducing agent. An electrical steel coated with this inorganic/organic insulative coating has good punching and welding quality. A disadvantage of this coating is that very high surface resistivity, a high stacking factor and acceptable weldability cannot be achieved simultaneously when the base metal surface roughness is greater than 20 Hr.m.s. .mu.inch. The coating has to be thick to insure good resistivity when applied to a rough sheet surface and this causes the stacking factor to be low and may adversely affect weld porosity. Another disadvantage of a coating having a resin with a particle size greater than 2 .mu.m is that the resin particles may detach from the steel surface during processing resulting in excessive powdering and powdered coating build-up. Also, this coating may contain chromic acid or chromates which create safety and environmental problems during application of the coating, processing of the coated steel sheet, and disposal of waste coating.
U.S. Pat. No. 4,618,377 discloses an inorganic/organic coating composition for forming an insulative coating on electrical steel comprising an organic emulsion resin, organic resin particles surface-treated with a dispersion improver, and a solution containing at least one inorganic phosphate or chromate component. The surface-treated resin particles are used to control surface roughness of the cured coating thereby improving weldability. The emulsion resin may be acrylic vinyl acetate, styrene, or butadiene alone or a copolymer of one or more of vinyl acetate, styrene, and butadiene. The surface-treated resin particles have a size of 2-50 .mu.m and may include a copolymer or mixture of one or more of polyethylene, polypropylene, polyamide, polyacrylic resin, polystyrene and benzoguanamine. The inorganic portion of the coating may include one or more of a phosphate of calcium, aluminum, magnesium and zinc, chromate or bichromate of calcium, magnesium, zinc or aluminum, and oxide, hydroxide or carbonate of calcium, magnesium, zinc and aluminum dissolved in phosphoric acid or anhydrous chromic acid. The coating is baked to semi-melt the resin particles thereby providing a surface roughness R.sub.a of 0.5-1.5 .mu.m on the cured insulative coating. A disadvantage of this coating is the resin particles tend to powder or flake during processing of the coated steel sheet. Also, the coating may contain chromic acid or chromates which cause safety and environmental problems.
U.S. Pat. No. 4,844,753 discloses an aqueous coating composition for forming an insulative coating on electrical steel comprising an inorganic chromate film-forming component and a resin component. The resin component is a mixture of an acrylic or acrylic-styrene emulsion and guanamine resin having a particle size of 0.2-1 .mu.m. When these inorganic/organic insulative coatings containing a chromate are applied with a very thin thickness, the coating is smooth but has poor surface resistivity. If the chromate insulative coating is applied with sufficient thickness to have high surface resistivity, excessive porosity occurs during welding caused by volatilization of the resin. If particles are added to the chromate insulative coating to minimize weld porosity caused by volatilization of the organic resin during welding, then the coating powders excessively during punching. This chromate insulative coating is undesirable because of safety and environmental problems relating to use of the coating and disposal of the waste by-products containing soluble hexavalent chromium.
It also is known to use insulative coatings for electrical steel containing as the binder one or more water soluble silicates such as sodium silicate, potassium silicate, or ammonium silicate. These coatings have a basic pH rather than the acid pH associated with coatings containing phosphate or chromate binders. Insulative coatings based on soluble silicates may be completely inorganic or they may contain organic material to improve punchability. U.S. Pat. No. 3,839,256 discloses a coating composition containing quaternary ammonium silicate solution and aqueous dispersions of ethylene polymers and ethylenically-unsaturated carboxylic acids or esters, and optionally containing lubricants such as oils to improve lubricity, surfactants to improve wetting or prevent foaming, and fillers to reduce cost. U.S. Pat. No. 4,507,360 discloses addition of a chromate compound selected from the group of strontium chromate, barium chromate, and lead chromate to the composition of U.S. Pat. No. 3,839,256 to improve corrosion resistance. U.S. Pat. No. 4,762,753 relates to an inorganic insulative coating composition for electrical steel containing sodium silicate, magnesium oxide or hydroxide, titanium dioxide, and mica. All of these insulative coatings based on soluble silicates provide good surface resistivity and heat resistance, but they cause poor weld pool fluidity during gas tungsten arc welding resulting in nonuniform, discontinuous welds.
Accordingly, there remains a need for an insulative coating for nonoriented electrical steel sheet that provides good surface resistivity, minimizes interlaminar power losses, has good water vapor absorption resistance, provides good adhesion to the sheet, causes minimal die wear during punching of laminations from the sheet, does not powder excessively during punching of the laminations or cause build-up of coating on the punching equipment, withstands stress-relief annealing and burn-off heat treatments, and does not cause excessive weld porosity. There also is a need for an insulative coating for nonoriented electrical steel sheet that is low cost, is compatible with a variety of chemicals and resins used to manufacture magnetic cores and does not contain chromates that create costly environmental disposal problems. There also is a need for an insulative coating for nonoriented electrical steel sheet that does not create a safety problem during application and use of the coating caused by the presence of hexavalent chromium.